The result of radiographic methods such as for example, computed tomography, mammography, angiography, X-ray inspection technology or comparable methods is, firstly, the display of the attenuation of an X-ray beam along its path from the X-ray source to the X-ray detector in a projection image. This attenuation is caused by the irradiated materials along the beam path, and so the attenuation can also be understood as a line integral over the attenuation coefficients of all the volume elements (voxels) along the beam path. Particularly in the case of tomography methods, for example in X-ray computed tomography (CT), it is possible to use reconstruction methods to calculate backward from the projected attenuation data to the attenuation coefficients μ(x) of the individual voxels x, and thus to arrive at a substantially more sensitive examination than with a consideration purely of projection images.
In order to display the attenuation distribution, the typical approach is generally to replace the physical linear attenuation coefficient μ(x) by the use of a value normalized to the attenuation coefficient of water, the so-called CT value. This is calculated from a linear attenuation coefficient μ(x) currently determined by measurement, and the reference attenuation coefficient μH2O, using the following equation:
      C    =                            μ          ⁡                      (            x            )                                    μ                      H            ⁢                                                  ⁢            2            ⁢            O                              -      1000        ,with the CT number C in Hounsfield Units [HU]. A value of CH2O=0 HU is returned for water, and a value of CL=−1000 HU is returned for air. Since the two displays can be transformed into one another or are equivalent, the term attenuation value generally selected below denotes both the linear attenuation coefficient μ(x) and an attenuation value dependent linearly thereon such as, for example, the CT value.
Modern tomography devices, such as, for example, X-ray computed tomography devices or C-arc units, are used for the recording and evaluation of image data in order to display the three-dimensional attenuation distribution. X-ray computed tomography devices generally have a recording system with an X-ray tube and a detector, lying opposite the latter, for detecting the radiation emanating from the X-ray tube and irradiating the object. The recording system rotates during the recording once or several times about the examination object. C-arc units, which are frequently used for imaging during surgical operations, comprise or two so-called C-arc systems as recording systems that are respectively rotated during the image data recording by an angle ≧180° about the object to be examined. The measured data supplied by the recording systems are further processed in an evaluation unit in order to obtain the desired sectional image or volume image of the examination area.
U.S. Pat. No. 4,991,190 A has also disclosed an X-ray computed tomography unit that has a number of recording systems that can revolve about a common rotation axis. The advantage of such tomography units with a number of recording systems as against a unit with only one recording system resides in the elevated data recording rate, which leads to a shorter recording time and/or an increased temporal resolution. A shortened recording time is advantageous because, in the reconstructed image, this results in the minimization of movement artifacts that can be caused, for example, by movement of the patient or of his organs such as, for example, the heart during the image data recording. An increased temporal resolution is required, for example, in order to display movement sequences when the data used for the reconstruction of an image need to be recorded in the shortest possible time. An imaging tomography unit with at least two recording systems is, for example, also disclosed in DE 103 02 565.
However, it is not possible to infer the material composition of an examination object from the attenuation value distribution of such X-ray recordings, since the X-ray absorption is determined both by the effective atomic number of the material and by the material density. Consequently, materials or tissue of different chemical as well as physical composition can exhibit identical attenuation values in the X-ray images.
In order to enhance the informativeness of an X-ray image based on the local attenuation coefficient, it is therefore known, for example from U.S. Pat. No. 4,247,774 A, to make use of X-ray spectra or X-ray quantum energies that differ from one another in order to generate an X-ray image. This method used in the field of computed tomography, which is also generally denoted as duel energy CT, makes use of the fact that materials of higher atomic number absorb low energy X-radiation substantially more strongly than materials of lower atomic number. In the case of higher X-ray energies, the attenuation values therefore match one another and are predominantly a function of the material density. By calculating the differences in the X-ray images recorded for different X-ray tube voltages, it is therefore possible to obtain additional information about the materials on which the individual image areas are based.
Even more specific statements are obtained when, in addition, the method of so-called base material decomposition is applied during X-ray imaging. In this method, the X-ray attenuation values of an examination object are measured with X-ray beams of lower and higher energy, and the values obtained are compared with the corresponding reference values of two base materials such as, for example, calcium for bone mineral (hydroxylapatite), and water for soft part tissues. It is assumed in this case that each measured value may be represented as a linear superposition of the measured values of the two base materials. Thus, a bone component and a soft tissue component can be calculated for each element of the pictorial display of the examination object from the comparison of the values of the base materials, such that it is then possible to transform the original recordings into displays of the two base materials.
Furthermore, the publication DE 101 43 131 B4 discloses a method that can be used to calculate the spatial distribution of the density ρ(r) and the effective atomic number Z(r) by evaluating the spectrally influenced measured data of an X-ray apparatus. The method is denoted as ρ-Z decomposition.
The evaluation of CT image data, recorded with the aid of two X-ray tube voltages, by means of photo/Compton effect decomposition, that is to say a decomposition by components of the photo effect and components of the Compton effect is known, for example, from the publication: Robert E. Alvarez and A Macovski, “Energy-selective Reconstruction in X-ray Computerized Tomography”, PHYS. MED. BIOL., 1976, vol. 21, No. 5, 733-744.
The recording of the CT image data with different spectral distributions that is required in the case of the three decomposition methods addressed is effected, for example, by operating the X-ray source of the recording system alternately with different tube voltages, or by providing two X-ray tubes that are operated synchronously with different tube voltages. The decomposition methods addressed can be used to identify different materials (for example soft part tissues, bone, contrast agent), and specifically reconstruct their spatial density distributions.
In the case of dual energy CT recordings, independent measurements are present in initial data records in conjunction with different X-ray tube voltages. For each voxel they supply two equations for two unknown parameters with the aid of which it is possible to characterize chemical material differences, for example.
For the further discussions,    Method 1: is understood as the base material decomposition, that is to say decomposition by components of two prescribable base materials (for example, plexiglass/aluminum; soft part/bone, water/iodine, etc.),    Method 2: is understood as the photo/Compton effect decomposition, that is to say decomposition by components of the photo effect and of the Compton effect on the linear attenuation coefficient, and    Method 3: is understood as the ρ-Z decomposition, that is to say representation of the density and the effective atomic number.
The best known methods are Methods 1 and 2, the base material decomposition being that most frequently applied owing to its graphic quality. Methods 1 and 2 lead to linear equations. Method 3 leads to nonlinear equations (compare in this regard B. J. Heismann et al., “Density and atomic number measurements with spectral X-ray absorption methods”, J. Applied Physics, Vol. 94(3), 2073-2079, (2003)). However, compared with Method 1 it has the advantage that it does not require any arbitrary advance stipulation of base materials. Said three methods are quantitative imaging methods. However, they can also be simplified to produce qualitative material classification. In this case, parameters derived from the measurements are used in order to assign the voxels of the image volume to various material classes.
The presence of two CT image data records in accordance with the two different X-ray tube voltages, yields a multiplicity of options for processing and visualization that support the posing of diagnostic questions. Apart from the dual energy material decomposition itself, every CT image recorded with the aid of different X-ray tube voltages is available.
The X-ray tube voltages in the case of dual energy imaging, for example at 80 kV and 140 kV, typically deviate substantially from the X-ray tube voltage of approximately 120 kV that is customary in the case of standard CT pictures. As a consequence thereof, the image impressions of visualized dual energy CT pictures likewise deviate, for example with regard to contrast and spatial resolution, from the image impression that is customary from standard CT pictures. On the part of the doctors who are accustomed to standard CT pictures, there is now a need to visualize dual energy CT image data in a customary form, that is to say in accordance with the X-ray tube voltage that is usual for standard CT pictures.